Modified barium titanate ceramic materials



April 17,v 1956 E. wAINI-:R

MODIFIED BARIUM TIIANAIE CERAMIC MATERIALS Original Filed April 10, 1951 a s an.. A@ i E,

lo la INI/ENTOR. EIIEENE-WEINER Eugene Wainer, cleveland migra, ontmanteling by Original application Apr-ii i0, n v

" lfnovv'atent No. 2,646,359, dated-duly 21, 1953.

lcause of these useful properties, barium titanateceramic's 2,742,376k i MODIFIED BARIUM rrrnNn'rn canarino ERIALS A .i1-corporation oliDelaware.Y f

iasaseriiu Ne. 220,162,

"Di- 1c vided and this 'application .Tinne $7,195.45, SerialzNo:

4amesne assignments, toV Radio Corporation for; America,

'l clams. (crinesay This application is `a division ot application, Serial No. 220,162.'I iiledApril'V 10, 1951, new U. S Llatent'HNo. A 2,646,359 issued July 2l, 19573.`

This invention relates'to novel ceramic materials having .high values of dielectric constant which remain relatively uniform over broad ranges of temperature.

More particularly, the invention relatesf to ceramic materials comprising barium titanate and minor amounts Vofcertain columbates and tantalates.

PRIOR ART i Barium titanate ceramics have previouslyfbcen'pre pared and `have been found to Vhave unusually highivalues `of-dielectric constant with relatively low losswlvalues.` Barium titanate ceramics have also been found to possess excellent piezoelectric properties if a polarizationavoltag'e is4 first applied across the ceramicfor ashort time. UfBe havebeenused as capacitors and as transducers.'

`A`t`actor which limits the use of barium t1tanate ce` r framics in capacitors, however, is vtheir relativelyxlow* Curie temperature or" about 116 C. and the factthat,fn v

the Curie fpoint region, the dielectric constant of barium 'titanate ceramics rises sharply to a very high peak. i Thus,- inthe temperature range of about SWU-150 C., vthe electrical properties-vary so greatly that'these ceramics can-3 not be used in many applications wheregoody stability is` required. 1

marium titanate ceramics Vhave also previously .been prepared with various other materials included. Among the materials which have been combined with barium titanate ceramics to modify their properties are: `titanium dioxide,`stanni coxide, alkaline earth stannates,flead titanate; magnesium titanate, calcium titanate, strontium titanate, `alkaline earth uor'ides, and alkaline earthkzirconates. l n i Y 'A' PRESENT INVENTION v ,The ,present invention is based on the discovery..that the l.addition `of minor amounts of any one ofsodium, potassium, or cadmium columoates or tantalates'to bari fium Ytitanate ceramic material resultsin the formation of novelceramicmaterials which have much greater stability of electrical properties over broad, practical temperature rangesthanbarium titanate, alone. The Cur iepoint peak in the curve of dielectric constant `of barium titanate .ceramic is almost entirely eliminated in the materials `ot the presentinvention. Moreover, the materials of the lpresent invention exhibitdielectric constants andxdissipa-V tion factors which are nearly as good asthoseofbarium titanate, alone. t Y p .The novel materialsof thepresent invention are vit?V .reous ceramics comprising a major proportion of barium titanate and from about 0.5 to about 50 percent by weight v @mi April 29, 1950` (and `now abandoned).

' 'vitrication l foBrECTs OFTHE INVENTION i 'Oner object ofthe present invention is to provide novel ceramic materials having good stability of electrical properties at .practical use temperatures.

Anotherobjectof the invention is to provide novel v ceramic materials having high dielectric constants which f remain substantially uniform over relatively :broad tem perature ranges.

VAnother object of the inventionis to provide an improved method of stabilizing` the dielectric properties of barium .titanateceramics f. t

S'tillr another object'of-the inventionis toV provide a method of substantially eliminating `the Curie point peak in the curve of dielectric constantnormallyassociated Y with barium titanate ceramics.

These andother objects will be moreapparent and the invention-will bemore readily understood-from the following ,detailed .description Vand `the accompanying drawing, Vof which:

.s Figuremlyis a graph showing variations-of dielectric constant with temperature for certain compositions of variousl materialsI within the present invention,

FigureaZ-is agraph showing variations of `dissipation factor with temperature for the same compositionsfor whichdata have Vbeen plotted on -the graph of Figure 1.

METHODS OF PREPARATION f `In general, the materials .of the presenLinVentionare prepared by, first, separately calciningV suitable 4quantities fsubstances which will separately form barium titanate and the particular one of the columbates -or tantalates to be added'. The titanate and the columbate or tantalate,

as .thecase-may be, are then homogeneously mixedA and fired to vitrication temperature for a brief timeginoxygen or air.

"" Examplel mixture was prepared consisting of percent :by weight barium titanate and 10 percent by weight, sodium columbate.r The preparation of sodiumk columbateand the other alkali metal columbates or tantalates,'as well as `thelalkaline earth metal columbates ortantalatesLin their ceramic form, has been fully disclosed in copending application, Serial No. 159,094, of Eugene' Wainer, kiled columbate may, tor example, be prepared by .mixing equimolar quantities of sodium` carbonate and vpure CbzOs and calcining at about l900 F. in an atmosphere of4 oxygen. The `oxygen ,mayv be either at atmospheric pressure or `any pressure above atmospheric. Firing-time during calcination should be as short as feasible, =usua1ly l/z ghour to 2 hours.l The mixture of the carbonate and a columbate may have incorporated with it about'lO percent by` weight of a 2 percent by weight aqueous solution of methyl cellulose as a temporary binder, and then be molded. to any desired shape for tiring. The molded body is -then red at about 2500 F. in air or oxygen for aboutl hour. The firing temperature is' not critical except thatA the temperature must be sufficiently high to vitrifyethe product. lt is desirable to use as low a temperature and as short a time as will produce uniform The ceramic body thus produced maybe aged by beingrpermitted to stand at roomtemperature for 2 weeks or more. The aging effect may be facilitated by annealing the body, after tiring, at a temperature of about 1000" F. for one hour.

Overallrange of firing temperatures suitable for firing anyof thematerials of the present invention is from about 2200 P. to about 2600 F. Optimum firing temperature for barium titanate, itself, is'about 245`02500 F. ..Asxthe amount of Vcolumbateor tantalate addition increases, theftemperature of vitritication decreases.

period, of

The sodium body composed of 50% barium titanate and 50% sodium columbate has a vitrication temperature of 23002400 F. Bodies having potassium columbate as the additive have still lower vitriiication temperatures, so that at about 50% addition of the potassium compound, vitrication temperature is 22002300 F. The same general result is obtained by adding cadmium columbate. A 20% addition of the cadmium compound results in a vitrication g temperature, in an oxygen atmosphere, of 2100-2200 F., the latter temperature being preferred. Using lesser amounts of cadmium columbate, the composition will Vitrify in the range of 23002500 F., with 2450 F.

` being the preferred temperature.

Although it is desirable to tire the bodies of the present invention at the lowest possiblev vitrification temperatures, care must be taken that the bodies are completely vitried. When the above ranges of temperature are used, both of these conditions are met.

Range of firing times is dependent upon the size of the bodies. Bodies having a thickness of less than 0.1 inch maybe completelyiired in a time period of 20-30 minutes. Bodies of the order of 0.1-0.3 inch in thickness requiring a tiring time of 30 minutes to l hour. If the thickness of the body is around 0.5 inch, a ring time of at least 2 hours should be provided.l

Air or oxygen firing atmospheres or mixtures of air and oxygen may be used but oxygen, alone, produces best results in all cases.

The annealing process should be carried out for periods of about one hour at temperatures of about 1000l200 F. Neither the Ytemperature nor the time is particularly critical, however. The principal requirement is that the temperature soaking be carried out for a suilicient time to aiect the whole body and that the annealing temperature Vbe lower than the vitriiicau'on temperature.

PROPERTIES OF THE PRESENT MATERIALS The following comparison may be made between the properties of a barium titanate ceramic without modifiers added and the ceramic material prepared as above described. Pure barium titanate ceramic has a dielectric constant in the range of about 1150-1600 over a temperature range of about 20 C. to 70 C. At about 70 C., both dielectric constant and power factor start to increase rapidly, the dielectric constant changing from an average value of about 1300 to about 7000 at a temperature of 110 C.-l20 C. Within this latter temperature range, Vthe body is very weak mechanically, due to the shifting from tetragonal to cubic structure. In view of the broad change in dielectric constant, characteristics of any circuit in which pure barium titanate capacitors are a part are drastically and disadvantageously changed if the ambient temperature is allowed to rise about 70 C. The material of the present invention, prepared as above described, on the other hand, has a dielectric constant of about 1250 at 20 C. and about 1350 at 100 C. Moreover, between 100 and 120 C., the dielectric constant varies only about y150 points. The variation of dielectric constant with temperature, between 20 and 120 C., for the material of the present invention, as described lin the above example, is shown in curve A of Figure 1. Its dielectric constant varies less than 25% between 20 C. and 130 C., compared to a variation of 60G-700 per cent for pure barium titanate. It will also be noted that, at room temperature, thedielectric constant of the material which consists of -90 percent by weight of barium titanate and 10 percent by weight of sodium columbate is nearly the same as that of pure barium titanate.

Curves B, C, and D of Figure l show the variation of dielectric constant with temperature, between about 20 C. and about 130 C., for the ceramic materials having the compositions 90 percent BaTiOs-IO percent KCbO3, 90 percent BaTiOs-l() percent NaTaO3, and 90 percent BaTiOs-lO percent KTaOa, respectively. In all cases, it has been found, as shown by the data plotted in Figure l, that the dielectric constant, within the temperature range chosen, remains relatively uniform.

Curves A', B', C', and D of Figure 2 show how the electrical dissipation factor varies with temperature for Vthe same compositions for which data are shown in Figure l. The material of curve A in Figure 2 is the same as that of curve A in Figure 1, etc. These data show that, in general, the dissipation factors of the materials of the present invention decrease with increasing temperature. 'Ihis is of considerable advantage for many applications where the temperature of a ceramic element rises when the element is in normal use.

Electrical properties such as capacitance, dielectric constant, and electrical dissipation factor, have been measured for many compositions of materials within the scope of the present invention and the values for some of these are found in Table 1 below. This table contains data only for those materials which comprise barium titanate and any one of sodium or potassium columbate or tantalate. Capacitance has been given in micromicrofarads.

TABLE 1 o Dpi n Composition ss p8" Dielectric mmfd. tioxolgac- Constant Bamm-100% 510 02 1, 401 99% Bevor-1% Naobot.-- 720 .51 1, 790 98% Baos-2% Neotion.-. 800 .5e 1, 700 95 25% Barron-1.75% Nacho 590 .31 1, 490 90% Barros-10% Naob0 450 07 1, 250 75% Bamm-25% Naobom- 545 1e 50% Bros-50% Nachos 388 .59

99% Bamm-1% Kobor 520 1. 04 1, 520 98% BaTioa-z 650 1. 0s

570 .42 51o .15 430 .11 310 .03a 230 .2s 110 .24

99% Barros-1% K'raoi 480 .43 98% Barrot-2% KTaoa 530 .51 95.25% Barros-4.75% KTaOa. 480 .10 90% Barrot-10% KTao 400 .05 Barrot-15% K'laoi. 35o .02 75% Hanoi-25% KTaos- 290 .50 50% BaT10;-50% K'raoa 305 .sa

The data shown in the above table indicate that the dielectric constant and the capacitance of barium titanate can be increased by adding small percentages of either sodium or `potassium columbate or tantalate. The minimum amount of the addition which is significant in changing the properties of the barium titanate is about 0.5%. maximum in the values of dielectric constant and capacitance appears to occur in ceramics having about 2 percent by Weight of the additive material. The materials which comprise 98 percent barium titanate and 2 percent of the additive by weight, therefore, are to be regarded as preferred embodiments of the materials within the present invention.

It will also be noted that a minimum in the dissipation factor value occurs where the constituents are present in the proportion of about percent barium titanate to l0 percent of the additive and these materials are also preferred Vfor some applications since, in all cases, they also have relatively high dielectric constants.

In general, the materials shown in the above table have relatively high dielectric constants at room temperature and also relatively uniform values of dielectric constant between 20 and 120 C. This is one of their most useful properties and illustrates one of the greatest improvements obtained by adding these particular columbates and tantalates to barium titanate ceramic. Y

It has also been found that cadmium columbate exerts s Y* a `.similar ei'ectlfonf 1b'ariu'ni:'titanatefcerarriictwherrtadiied to the titanate in amounts up to 20 percent.byweight. A series of 'l compositions was preparedun which: the relative amountsof-fbarium titanate and Acadmium columbate or tantalate were varied. The .resultsfiofmleasurements made of certain electricalA propertiesof those materials in which the columbate was included are given in Table 2 below:

Y TABLE 2 y [BaTlOs-CdCbOa Fired in an Oxygen Atmosphere] TFiring Before Aging After Aging Empel'- Composltion mure,

F. E D E D 1. 100% Bs'lioa 2,450 1,320 0.05 1, 330 0. 04V 2. 09% Barros-1% 000003 2, 450 1, 200 0. 20 1, 205 0. 0s 3.08 Barr o; 2%

dCboi 2, 450 1, 220 o. as 1, 240 0. 12 l4. 05.25% Beffroi-4.15%

odobos 2, 425 1, 040 0. 34 1,140 0.10 5. 00.0% Barros-9.1%

odCb03.-- 2, 400 100 0.10 1,000 0.04 0. s1 Ba'ri -1a f 7i1obo3... 3 2, 350 530 0. 09 050 0. 03 7. 83.4 BaTiO 10.0

C l17b03...--. Z 2,100 440 l 0. 30 550 0.04

In those ceramics which comprise barium titanate and cadmium columbate, a maximum dielectric constant 1s obtained by the addition of about 1V percent of the columbate; Dissipation factor is lowest for those materials con 'taining about 10 percent to 15 percent cadmium colum-V l .was sodium or potassium columbate or tantalate, the materials formed by addition of cadmium columbate or tantalate show eliminationv of the high temperature Curie point transformation characteristic of barium titanate.

Consequently, the temperature range of usefulness of these materials as capacitors with high dielectric constant is broadened considerably relative to that of barium titanate alone. To take some specific examples, the material which includes l percent by weight of cadmium columbate has a dielectric constant which drops slowly from a value of 1265 at 20 C. to 1200 at 65 C. and then rises uniformly to a Curie point `peak of 3200 at 114 C. This may be compared with a normal Curie point peak for barium titanate which is of the order of 6000. For a composition containing about percent cadmium columbate, Vthe dielectric constant is quite uniformfrom 10 C. to 100 C., having a value of about 1000, and a slight Curie point maximum of 1400 at 115 C. For this material, the dissipation factor decreases uniformly, going througha slight maximum equivalent to a value yof .115 at 38 C., the value dropping to .02 at 115 C. A material which contains about 20 percent cadmium columbate has a dielectric constant which remains quite uniform at about 550 from 20 C. to 120 C., showing vonly a slight rise to a value of about'600 at 115 C. The dissipation factor, however, shows a sharp maximum of .22 at 14 C. and this factor decreases regularly to .022 at 120 C.

The materials comprising barium titanate andocadmium columbate may be prepared by the general method described for the sodiumor potassium columbate or tantalate addition materials, as described'in Example 1. A

more detailed description of the preparation of cadmium columbate or tantalate is found in a copending application of Samuel Bousky, Serial No. 229,533, led June 1, 1951 (and now abandoned).

An example of preparation will now be given:

Example Il caloijnde .infairoroxygenzat aftemperature of-ab'out 2100 l example, 90 percent by weight of the former and 10 percent by weight of the latter. The mixture is ground to an impalpable powder, al temporary binder such as a water solution of polyvinyl alcohol or methyl cellulose is added, the material is molded into a shape convenient for firing, and the body is redin oxygen or air to the temperature indicated in Table 2,and maintained at the y firing temperature for a length of time suicient to develop complete vitrication.

When initially made, the electrical properties of these bodies are relatively poor but, after standing at room temperature for 2 weeks to a month, the dielectric constants tend to increase and the dissipation factors decrease considerably. This aging eifect may be greatly speeded up about 1000 F. for at least 1/2 hour.

In connection with the values for dissipation factor given in theY above tables it is to be understood that this property is a highly variable quantity sharply affected by humidity conditions prevailing at the time of measurement, slight changes, in mode of tiring, and variation in application of electrodes. Generally speaking, a precise value of dissipation factor for a particular dielectric can be obtained only by rigidly controlling all variables in manufacture and assembly which might affect the properties. It is usually safer to use an average value measured on many bodies than a single value measured on any one sample. i i

In addition to the useful electrical properties mentioned previously, the ceramic materials comprising barium titanate and sodium columbate or tantalate have been found to exhibit ferroelectric and piezoelectric properties.

In general, the materials of the present invention are useful as the ceramic dielectric portions of capacitors used as bypass elements in communications circuits where the temperature tends' to rise unduly. Excess temperature Vrise may be caused by high power dissiptation in the circuit as a whole, or crowding together of vacuum tubes and capacitors as a result of miniaturization. The trend in miniaturization of circuits has resulted in greatly increased demand for circuit elements which will retain stable operation over broad temperature ranges.

Certain high dielectric ceramic materials now in use, such as unmodified barium titanate, maintain their rated capacitance, dissiptation, and breakdown values up to about C. If the temperature rises substantially higherl than' this, marked changes in these properties occur resulting in failure of circuit elements.

Materials of the present invention maintain substantially their rated electrical values up to about C. and their dissipation and breakdown characteristics improve with rise in temperature.

What is claimed is:

l. A vitreous ceramic material comprising barium titanate and a substance selected from the class consisting ofcadmium columbates and tantalates, the percentage by weight of said cadmium compounds being within the range of about 0.5 to about 20.

2. A material according to claim 1 in which said substance is cadmium columbate.

' 3. A material according to claim 2 in which the per- I dielectric constant body comprising mixing together v powdered barium titanate vand a substance selected from Y the class consisting of cadmium columbates and tantalates,

being within the range of about 0.5 and about 20, molding 6v. v A method according to claim 4 in which said annosl ^s`aid mixture into a coherent body, and ring said body phere is air. v in an oxidizing atmosphere at temperatures between about 7 A method aCCOl'dlHg t0 Clalm 4 1U WhlCh Said body 2100s and 25000 E for at least 20 minutes is. anneaied, after firing, at a temperature lower than the 5. A method according to claim 4 in which said atmos- 5 Vmcauon temperature' phere `consists essentially of oxygen. y No referencescited. 

1. A VITREOUS CERAMIC MATERIAL COMPRISING BARIUM TITANATE AND A SUBSTANCE SELECTED FROM THE CLASS CONSISTING OF CADMIUM COLUMBATES AND TANTALATES, THE PERCENTAGE BY WEIGHT OF SAID CADMIUM COMPOUNDS BEING WITHIN THE RANGE OF ABOUT 0:5 TO ABOUT
 20. 